E2E tests: Wait for requests after adding suggestion

What does this MR do and why?

Resolves #377680 (closed) Resolves #366447 (closed)

For more context #377680 (comment 1211724650)

In the browser_ui/3_create/merge_request/suggestions/* E2E tests we log in as one user to make the suggestion, and then we log in as another user to accept the suggestion.

Occasionally we get a 422 when clicking on the sign in button when switching users. This is reproducible locally with the GDK about 10% of the time if you continually run the test.

The solution is to wait for requests to finish before clicking sign in. When timing wait_for_requests there was evidence of requests still running when clicking sign in. After this change I no longer received the 422. (tested with many test runs)

The same issue was also discovered and fixed in the feature specs: !101276 (diffs)

How to set up and validate locally

bundle exec rspec --order defined qa/specs/features/browser_ui/3_create/merge_request/suggestions/custom_commit_suggestion_spec.rb

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